This invention relates to a new lubricant and a method of lubricating an assembly which utilizes this lubricant. The lubricant comprises a mixture of a binder and a lubricating agent. In the method, the binder has a melting point below the temperature of the surface onto which the lubricant is applied so that, on application, the binder melts to form a film on the surface containing the lubricating agent. On cooling the binder solidifies encapsulating the lubricating agent and forming the lubricant on the surface for further processing steps.
This lubricant and method have been found to be particularly useful in manufacturing processes relating to rubber goods, specifically tubes for pneumatic tires and the like. By way of example, in the manufacturing process for rubber goods, rubber compounds containing the rubber polymer, either natural rubber, synthetic rubber, or combinations thereof, along with reinforcing agents, softeners, antioxidants, vulcanization accelerators and vulcanization agents are mixed to form an unvulcanized rubber compound. The rubber compound may then be formed into various preassemblies which are incorporated into the final product or directly to a final product. The final product is then normally placed into a mold and subjected to vulcanization temperatures resulting in the final, vulcanized end product. In many of these processes it is necessary to handle the unvulcanized rubber compound through several steps and to store the unvulcanized rubber compound between the processing steps.
An unvulcanized rubber compound is normally tacky or sticky so that when its surfaces contact other materials or other parts of the preassemblies, the surfaces will adhere and create a problem. It is, therefore, necessary to have a lubricant material that will prevent the sticking of the unvulcanized rubber assembly to itself or to other surfaces.
In the manufacture of tubes for pneumatic tires, the ingredients in an unvulcanized rubber compound are mixed to form the rubber compound. The rubber compound is then tubed or extruded. In this process, the rubber compound is made plastic by working it on a mill which thereby heats the rubber compound. The warm rubber compound is then passed through the tuber or extruder. The tuber may be thought of as a big meat grinder with a rotating screw in a barrel. The rubber compound is forced out of the end or head of the barrel through a die to form a predetermined shape by the pressure of the screw. This tubing operation is standard in the manufacture of many rubber goods.
This invention is applicable to the hot, tubed shape that results from the tubing operation. After the tubed shape is formed, the lubricant of this invention is applied to the surface of the shape. The surface of the shape has a temperature higher than the melting point of the binder material so that the binder melts on contact with the hot surface, thereby forming a film which contains the lubricating agent. The binder solidifies on cooling encapsulating the lubricating agent on the surface.
More specifically, in the manufacture of a tube for a pneumatic tire, the shape that exits from the die at the end of the tuber is a continuous cylinder or annular piece of uncured rubber compound. The lubricant is applied to the inner surface of the tube by a mechanical dispenser through a pipe connected to the "spider" and shaft of the tuber located in the tuber head. It is applied to the external surface of the tube by standard, known mechanical means. In this manner all of the surfaces of the tube, both the inner surfaces and the external surfaces, are lubricated so that they do not stick to one another. In the further manufacturing steps the continuous cylinder is cut to length for tubes, the ends spliced together, the valve is affixed and the resulting annular member is vulcanized in an appropriate mold to form the standard, tire innertube.
Prior lubricating materials utilized in the manufacture of rubber goods have comprised basically two types; lubricating agents in a powder form or lubricating agents in a liquid form. In the powder form lubricating agents such as talc (soapstone) have been mechanically applied to uncured rubber surfaces. Due to the powdery or dusty nature of these materials, severe problems have been encountered with the application of these materials due to their contamination of the ambient air (a dust problem), non-uniformity in the application of the material, non-uniformity in the material remaining on the surface after application and, lastly, the fact that these materials often cause contamination in the end products.
Attempts to use liquid lubricating agents have been undertaken to combat these problems, particularly the dusting problem. However, the liquid agents create a drying problem that is just as bothersome as the dusting problem. If the liquid lubricating agent has not sufficiently dried before the product is processed further (particularly vulcanized) faulty products will result.